Quick and accurate particle diagnostic capabilities have become increasingly important in identifying and quantifying biological/environmental contaminants and other biological assays, such as anthrax spores in air or reticulocytes in blood. Numerous and varied techniques currently available require precise alignment of independent components, which are typically mounted in a laboratory in a semi-permanent fashion. Portable gas chromatographs/mass spectrometers (GC/MS) remain expensive and often overly complicated for field personnel, who must master numerous and varied equipment. Flow cytometers for blood analysis are more commonplace, but typically employ an expensive argon laser to produce a necessary blue light source, and cannot detect particles in air.
The stimulated Raman effect has been used to identify particular particles or classes thereof. As used herein, the term particle may refer to an individual molecule in addition to its more generic meaning. In general, the spontaneous or stimulated Raman effect begins with a molecule at an initial energy state that then absorbs incident energy, such as monochromatic light in the visible, infrared (IR), or ultraviolet (UV) regions. The energy absorption causes the molecule to temporarily enter an intermediate energy state, and eventually decay back to a resting energy state that differs from the initial energy state. Because of this difference, re-emitted energy (energy emitted on the transition from intermediate to resting states) from the molecule defines a spectral shift over the incident energy. Distribution of this re-emitted energy is known as Raman scattering. Raman spectroscopy, which includes Stokes and anti-Stokes transitions (positive or negative difference between initial and resting energy), may be used to identify the molecule or its characteristics based on the above spectral shift. Rayleigh scattering and Brillouin scattering may be similarly used to detect and identify a particle based on re-emitted energy. When the decay time from intermediate to resting energy state is instantaneous (˜10−7 seconds), the resulting energy is termed fluorescence. Energy from non-instantaneous decay times is termed phosphorescence.
Detection of airborne biological particles/molecules based on light scattering techniques, as opposed to Raman scattering wherein absorbed energy is re-emitted rather than merely scattered elastically, typically employs a large UV laser whose beam is focused into a single spot within an interrogation chamber into which particles are injected within an air sample. The particle injection takes place through a nozzle that defines a jet stream of carrier gas (e.g. air) within which the particles are flowing along a straight-line trajectory. See, for example, U.S. Pat. No. 5,133,602. Light scattering techniques may be used to detect particles based on size, but appear unable to distinguish between similarly sized particles. Light scattering is particularly useful as a ‘trigger’ signal to detect when a particle enters a detection or identification chamber. Light scattered from the entering particle may be used to activate an identification sensing system, which uniquely identifies the particle as one of interest or not. That is, the light scattering ‘trigger’ signal need not distinguish between an anthrax particle and an innocuous dust particle.
Conversely, fluorescence-based optical diagnostic techniques for airborne particles, as well as biologically/chemically specific species identification, provides a potentially efficient method to identify presence or absence of specific biological or particulate targets. Practical limitations on equipment used in Raman and light scattering spectroscopy have limited its application mostly to fixed laboratories. Raman scattering systems may uniquely identify a particle based on natural fluorescence of a bioagent or fluorescence of a protein that uniquely binds to a target bioagent. Some prior art Raman scattering systems employ a helium-neon, argon, or krypton laser, a double-scanning monochromator, a focusing lens system, and photon counting electronics. See, for example, OPTICS, Eugene Hecht, 4th Ed., pages 603–605 and FIG. 13.21. The monochromator is required to distinguish between incident energy and re-emitted energy, since unshifted laser light would be scattered along with the Raman spectra. Other prior art systems relied upon a resonant chamber to reflect a laser beam across the target molecule, stimulating more energy transitions that could be measured more reliably. Each of these prior art systems relies upon a high-powered laser to instantaneously (i.e., a single laser pulse or similar time period of target exposure to laser energy) induce fluorescence from the subject particle. Additionally, the related optics appears too sensitive incompatible with the rigors of field detection.
What is needed in the art is a portable optical particle diagnostic system for qualitative identification of target particles and/or molecules of interest, especially biological contaminants.